In recent years, lithium ion secondary batteries (hereinafter, also simply referred to as batteries) able to be charged and discharged are utilized as power sources for driving a hybrid electric vehicle and a portable electronic device such as a notebook-sized personal computer and a video camcorder.
One example of an electrode body of such a battery is a flat wound electrode body wound to be flat in transverse section about a winding axis. This electrode body includes a central portion located at a center of the transverse section in a long-side direction and end portions located respectively at both ends in the long-side direction.
In the lithium ion secondary battery, meanwhile, a positive electrode sheet and a negative electrode sheet constituting an electrode body will expand and contract in association with charge and discharge. Accordingly, the size or dimension of a battery case accommodating the electrode body changes. To suppress variations in battery performance due to such dimensional changes, in many cases, the central portion of the flat wound electrode body is compressed from outside of the battery case.
For instance, Patent Document 1 discloses a lithium secondary battery (a lithium ion secondary battery) configured such that an electrode group or assembly (a flat wound electrode body) is contained in a battery container (a battery case) having flat surface portions and side surface portions. In this lithium secondary battery, the thickness of each flat surface portion is designed to be larger than that of the side surface portions so that the flat surface portions of the battery container (the battery case) can press a flattened portion (a central portion) of the central portion of the electrode assembly (the flat wound electrode body).
Meanwhile, in a battery including an electrode assembly (a flat wound electrode body) described as a related art in Patent Document 1, (see FIG. 8 of Patent Document 1), the degree of strain on the flattened portion of the central portion (the central portion) of the electrode assembly is lower than that on curved portions (the end portions) located at both ends in the long-side direction of the transverse section of the electrode assembly. Accordingly, as shown in FIG. 8(b) of Patent Document 1, it is assumed that the electrode assembly may bulge inward. This is because a winding core of the electrode assembly is an air core and this electrode assembly is not fully compressed in a short-side direction of the transverse section, and thus no pressure is exerted on the flattened portion of this electrode assembly. The flattened portion with a relatively low strain is liable to generate gaps causing inward bulging than the curved portions.
On the other hand, in a battery described in embodiments of Patent Document 1 (see FIGS. 1 to 5 of Patent Document 1), with the use of a cylindrical winding core 6, the flattened portion of the central portion of the electrode assembly is pressed by the flat surface portions of the battery container (the battery case). Thus, the degree of strain on the flattened portion of the central portion becomes higher than other portions of the electrode assembly. Accordingly, the degree of strain between the flattened portion of the central portion of the electrode assembly and the curved portions are uniformized. As a result, it seems possible to prevent the generation of gaps in the electrode assembly.